We seek to elucidate mechanisms of photoreception in photobiological phenomena which respond with a characteristic flavin type action spectrum. We take our previous results showing that a flavin chromophore will mediate the photoreduction of a cytochrome b in vivo to indicate that similar light-induced redox changes are involved in the photoreception mechanisms of physiological blue-light photoresponses. We are currently studying the blue-light photoactivation of an enzyme nitrate reductase which contains both flavin and cytochrome b and is activated in vitro by irradiation with blue light.